A pyridoxal-phosphate protein [4]. In archaea and eukarya selenocysteine formation is achieved by a two-step process: EC 2.7.1.164 (O-phosphoseryl-tRNASec kinase) phosphorylates the endogenous L-seryl-tRNASec to O-phospho-L-seryl-tRNASec, and then this misacylated amino acid-tRNA species is converted to L-selenocysteinyl-tRNASec by Sep-tRNA:Sec-tRNA synthase.